1. Field of the Invention
The present invention relates to an optical waveguide and a method of manufacturing the same. In particular, it relates to a curved optical waveguide which connects two points for which the optical axes of the incident ray and the outgoing ray are not identical, and a method of manufacturing such waveguide.
2. Description of the Related Art
Optical components such as optical couplers, optical star couplers and optical modules are relevant to the field of optical LANs, telecommunication and optical instrumentation control, etc. In recent years, these optical components have been widening their field of application at a rapid pace, and, corresponding with this movement, the demand for compact components with high efficiency is becoming greater. In connection with the optical components above, optical waveguides are utilized for branching or combining optical signals, or for connection with light-emitting or light-receiving devices such as LD, LED and PD. Optical waveguides are composed of a core with high refractive index for light propagation, and a cladding with low refractive index which surrounds the core. They are characterized in that the aimed purposes can be achieved through adequate designing of the optical waveguide patterns of the core and the cladding.
A generally adopted method for using optical waveguides for guiding light from one point (P) to another point (Q) is to connect these two points by using curved waveguides to keep the loss in the waveguides as low as possible in cases where the propagating directions of the incident light at point P and the outgoing light at point Q are not identical. For example, by employing a method of connecting the arced portion and straight portion so that the direction of the tangent at an arbitrary point along the optical waveguide between points P and Q and the direction of the light propagation are identical, it is possible to determine a most preferable shape of the optical waveguide. Therefore, one optical waveguide with the shape determined by the above method has been conventionally used.
However, when using said curved optical waveguide, there is a curvature loss, a loss peculiar to the curved portion. A resulting problem is that the loss becomes greater with optical waveguides which are curved at acute angles.
Now, the curvature loss will be described with reference to the drawings. The explanation in the specification below relates only to optical waveguides with rectangular-shaped cross-sections, and the drawings are planar views. In order to simplify the explanations, only light which proceeds parallel to the planar face of the optical waveguide will be considered.
FIGS. 3(a) and 3(b) indicate the directions of light propagation at two curved optical waveguides which both have the same width but different curvature radiuses.
In FIG. 3(a), the incident ray in the core at point A reaches point B on the interface between the core and the clad, and is thereafter either completely outgoing into the core, or a portion of the light penetrates into the clad and incurs light leakage.
On the other hand, in FIG. 3(b), the incident ray in the core at point E reaches point F on the interface between the core and the clad, and is thereafter either completely outgoing into the core, or a portion of the light penetrates into the clad and incurs light leakage.
In both FIGS. 3(a) and 3(b), if said light is completely outgoing into the core, there is no loss, but if light leaks, this becomes the curvature loss mentioned above. Whether said light will be completely outgoing into the core or will leak is determined by the angle .alpha. (.beta.) formed by the tangent of the interface at point B (F) and the line portion AB (EF). If angle .alpha. (.beta.) is smaller than the critical angle of incidence, complete reflection occurs. If angle .alpha. (.beta.) is larger than the critical angle, a portion of the light penetrates into the cladding and leaks. The critical angle is hereby indicated as per the following formula: EQU Critical angle=cos.sup.-1 (n.sub.1 /n.sub.2)
n.sub.1 is the refractive index of the cladding and n.sub.2 is the refractive index of the core.
As shown by FIGS. 3(a) and 3(b), .alpha.&lt;.beta., and it is clear that the sharply curved waveguide (in FIG. 3(b)) is more likely to incur light leakage as the angle formed by the tangent of said interface and the line portion is greater. Provided that the relation .alpha.&lt;critical angle &lt;.beta. is true, light will disseminate as shown by the arrows in FIG. 3. In other words, whereas in FIG. 3(a) said light is totally reflected, there is light leakage in FIG. 3(b). As a result, when arbitrarily setting the position of point A (E) or the direction of the light propagation from point A (E), the sharper the curve, the greater the rate of light leakage in the curved optical waveguide.
With respect to optical waveguides with curved portions of the same radius of curvature but different widths, taking as examples the waveguide in FIG. 3(a) with its left side shown by solid lines and the same waveguide with its left side shown by dotted lines (going through point C), the range of the point of incidence (range of point A) of the wider optical waveguide can be considered to have shifted in a direction in which the light leakage increases, so that the greater the width of the optical waveguide, the greater the loss. In this way, waveguides having curved portions inevitably incur loss due to the curvature. However, the degree of loss differs with the waveguide width (D) at the curved portion and the radius (R) of the curvature, depending approximately on the ratio R/D. The smaller the R/D, the greater the loss based on the curvature. Furthermore, when determining D and R, the appropriate range for the waveguide width D is determined among others as a consequence of the core radius of the optical fiber to be connected with this optical waveguide, so that the curvature radius R is important in deciding the shape of the optical waveguide.
Recently, demands for compact optical components such as optical couplers are becoming greater, so that optical waveguides also need to be made as compact as possible to achieve this purpose. In order to make waveguides more compact, it is vital to shorten the length of the optical waveguide, and there arises the necessity to connect two points at a short distance with differing directions of the incident ray and the outgoing ray via an optical waveguide, which inevitably brings along sharply curved optical waveguides. As described above, sharply curved portions greatly increase curvature loss and make it difficult to obtain optical waveguides which satisfy the required performance.